Frequency modulator



April 14, 1942. M. G. CROSBY' A FREQUENCY MoDULAToR 2 Shets-Sheet 1 Filed April 13, `193'( y@ m ATTORNEY Patented pr.`14, 1942 FREQUENCY MoDULA'ron Mun-ay G. Crosby, Riverhead, N. Y., assignmi6 Radio Corporation of America, a corporation of Delaware Application April 1s, 1937, seriali No. '136,578

47 claims. (cl. 17a-'1715i This invention discloses a .type of frequency modulator wherein a. high degree of stability is maintained by the use of automatic frequency control. The same controlling means is used for automatically controlling the mean frequency of the modulation as is used to produce the frequency modulation. In this way -a Sim- "ple frequency modulator with a high degree of stability is obtained without the necessity of a high degree of frequency multiplication.

In the prior art of generating frequency modcomparable to a crystal controlled oscillator and yet the frequency of the oscillator can be varied n. over `a wide rangeof frequencies linearly in accordance with modulating potentials. Due to ulation, one method used toobtain frequency modulation without sacrificing stability has been the use of phase modulation with correction or modification of the audio frequency modulating potentials and the use f a high degree of frequency multiplication. Since the amount of phase modulation which may be obtained in a phase modulator is effectively a very small amount of frequency modulation, a high degree of frequency multiplication is necessary to obtain frequency modulation by a phase modulation and multiplication method. Also, the audio correction necessary to correct the phase modulation to frequency modulation involves the use of correction circuits which necessitates addithe wide frequency modulation which canbe obtained in the modulated oscillator less frequency multiplication is required to produce a Y high degree of frequency modulation.

In`the frequency modulator of this invention automatic frequency control isused to hold the mean frequency of the modulated oscillator at a fixed position. The oscillator energy is heterodyned to an intermediate frequency by means of a crystal oscillator' whose stability is veryhigh. This intermediatev frequency is passed through frequency discriminating circuits and rectified so that the rectified energy may be used tovary an element voltage of a "reactance tube connected to the tuned circuit of the modulated oscillator.

tional amph'cation to make up'for' the losses induced by the correction circuits.

When the ordinary oscillator, utilizing tuned l circuits, is frequency modulated, in order to obtain a high degree of frequency modulation Without encountering inertia or ily-wheel effects, the tuned circuit of the oscillator must be damped or constructed with high losses so that the frequency may be modulated at a high rate without distortion to the higher modulation frequencies. When this damping is applied to the oscillator tuned circuit,'or when low Q circuits are vused for the tuned circuit, the stability of the mean frequency of the oscillator is reduced in accordance with the reduction `iri Q of the tuned circuit. Thus, the required damping of the tuned circuit loosens up the stability of the mean frequency so that when the oscillator is frequency modulated, the degree of modulation is dependent upon the degree of stability present. Hence, mean frequency instabilityJ is attendant to a -high degree of frequency modulation.V

My frequency modulator utilizes automatic y frequency control of a frequency modulated oscillator to insure a high degree of stability for slow frequency variations or drift but its stability is loosened for frequency variations at signal frequency. In this manner I provide a simple frequency modulated oscillator with stability 55.

Thus, mean frequency variations of the oscillator are converted to' energy which operates a reactance in the said oscillator circuit in a manner toV compensate for said mean frequency variation tendencies. These frequency variation tendencies occur at rates which are slow compared to modulation frequencies ordinarily used for signalling. Hence, signalling frequencies may also be applied to an element voltage of the reactance tube which will modulate the frequency of the oscillator but which will not be compensated by the automatic frequency control circuits due to time constant circuits placed in the automatic frequency controlling circuits.

In describing myA invention, reference Will be made to the attached drawings wherein:

Figures l and 2 each show a frequency modulator circuit wherein the Ymean frequency ofthe oscillator isv maintained constant and wherein the frequency of the oscillator is modulatedin accordance with signals or other controlling potentials; while l l Figure 3 is a vector diagram illustrating the manner in which the frequency controlling potentials are obtained.

Figure 1 shows a specific embodimento the principle employed in this invention. lTube 'I0 is the oscillator tube, tube 20 is the reactance tube,

tube is the frequency converter tube and ltubes t0 and 4I are diodes which have been shown as separate tubes but maybe enclosed in a-single envelope. The oscillator I0 having tuned circuitv i3 is of the grounded grid typ and has its output coupled to power amplifier 5 and through 5 to antenna 6. Unit 5 may a1so.contain amplitude limiters and frequency multipliers. Partei the` by piezo-electric crystal I4and tuned circuit-'I4' to the cathode and operated in a well known manner to produce oscillations of a frequencyV determined by I4 and I4. Preferably the circuit I4' is tunedto be inductively reactive to the oscillations produced. The oscillations produced are impressed on or modulate the electron stream of the tube and reach the shielded anode 23 by Way of the electron stream only. Crystal I4, in

conjunction with anode tuned circuit I4', oscillates at a frequency differing from the frequencyl of tuned circuit lI3 by a frequency difference lequal to an intermediate frequency.l Thus, the electron stream of tube 30 is controlled by oscillations produced in 30 and by oscillations from I0. The transformer consisting of primary and secondary windings I1 and I8 is tuned to the said intermediate frequency. The mid-tap of the secondary I8 of this transformer is connected through a by-pass and coupling condenser C to I.

the high intermediate f1 equency potential side of the primary winding I1 of the transformer. Thus, the voltage applied to each of the diodes and 4I is the combination of the voltage applied to the high potential end of primary winding I1 and the voltage induced by winding I1 in the secondary winding I8. The transformer I1, I8 forms a retard circuit so that the voltage fed to 'the diodes 40 and 4I is a combination or resultantv of a retarded voltage and an unretarded voltage. Thus, this coupling forms a frequency modulation sloping lter in the manner described in the Crosby United States application #618,154 filed June 20, 1932, Patent No. 2,229,640, January 28, 1941. In the arrangement of the said Crosby application the retarded voltage is combined ,with the unretarded voltage by meansA6 of coupling tubes. In the circuit of Figure 1 of this disclosure, coupling tubes are dispensed with and the retarded voltage `appearing at the transformer secondary I8 is combined directly with the unretarded voltage appearing at its 'primary I1 and supplied to I8 through coupling condenser C. The energy from the frequency discriminat- 'ing transformer I1 and I8 is differentilly-'rectified by diodes 40 and 4I. The rectifled energy appears in resistors 24 and 26 and a voltage char- -energy from the oscillator circuit I3 is fed to one plied frequency varies. Thus, if the primary and secondary voltages are added vectorlally, the absolute magnitude of the, resultant vector will be greater on one side of resonance than on the other. The potentials at the ends of the inductance of I8, i. e., at X and Y are 180 out of phase because the center point is connected to the primary I1. Thus two potentiaisfimay be produced, one maximizing above and one maximizing below the center or resonant frequency. These voltages are applied to the rectiers 40 and 4|. r

In Figure 3 let P represent the primary voltage supplied by C to the anodes of 40 and 4I. Let S represent the voltage induced in I8 and assume .that the-correct intermediate frequency is set up in the primary I1 so that an exact phase quadrature relation is obtainedbetween the primary voltage P and secondary voltage S. Two resultants R40 and R41 are produced and act through 40- and 4I to produce in 24 and 26 opposite and equal currents and no potential is supplied by .40, 4I to control the reactance tube 20 and in turn the tune of I3. Under this condition the tuning of I3 is adjusted to produce in I0 the frequency desired as the mean frequency of the frequency modulated transmitter.

VNow assume that the mean intermediate f requency slowly changes due to shift in frequency of III. The phase relation between.P and S heretofore 90 changes and S takes up a new position as shown at S in Figure 3. The 180 phase relation between the secondary voltage at points X and Y remains. The vector R'u has become longer while the vector Rm has become shorter: Thus, the voltage onV 4I hasbecome greater and that on 40 smaller so that a resultant is produced at 24 and 26 to act through R1 to retune I3 in a direction to returnthe 40 (intermediate frequency-to the correct value. If

the mean frequency shifts in the other direction the vector relation of' P and S changes and the. voltage applied to 40 becomes greater than that applied to 4I and I3 is tuned in the opposite direction. In other words, if the proper mean frequency is applied-to I1 equal voltages are applied between X and ground and Y and ground.

, These voltages are rectified in 40 and 4I and acteristic thereof is passed through time constant elements RiCi, or R102, depending on' theY position of switch S, and thence throughtransformer sec# ondary 29 and through grid leak 21-to the grid of the reactance tube 20. Y f

The operation -of the'frequency discriminating circuit comprising the intermediate frequency transformer I1 and I8 andthe direct coupling C Vto produce a potential of a value characteristic of the extent of deviation of the mean intermediate frequency from its correct value and of a polarity characteristic of the direction of said deviation from said correct value will now be described. The differential direct current potentials produced in 24 and 26 whose magnitude and polarity are determined by the amount and sign, respectively, of the difference between the applied fre-- quency in I1 and the induced frequency in I8 is obtained -byrectifying action of 40 and 4I. As is known, a 90- phase relation exists between the primary voltage in I1 and the secondary voltage in I8 where I 1 and I8 are loosely coupled and both tuned. This phase relation changeszas the apappear in 24 and 26 flowing in opposite directions. vThe potentials-are equal and the net is zero potential between Z and ground. However, whenthe applied frequency at I1 is incorrect and departs from resonance the frequency discriminating circuit comes into play land the voltagesl between X and ground and Y and lground are unequal. )Unequal drops in 24 and 26 produce a resultant potential at Z which acts on 21. The .polarity of this potential depends on direction of frequency departure from center frequency. C4 and Cs are of low impedance to the intermediate frequency.

Reactance tube '20 is if the type in which.

a vacuum tube is converted into a reactance by 4feeding an out of phase voltage to its grid. Thus, resistor Rz and condenser C3 form a phase shiftingcircuit which shifts the v oltage supplied from the anode 3| of I0 to the plate 33 ofthe reactance tube 20 so that itv reaches thelgrid 28 of tube 20 with a 90 phase lag. The value of R; ismade high as comparedrto the reactance of Ca rfor the frequency used so that the current through the circuit is largely resistive and is in phase with the voltage.l However, the volt- -age drop through C3 will lead the current bysubstantially and the desired phase' relation necessary to produce the reactive effect in 20 is obtained.

The control tube 20 acts to vary the oscillator frequency and this variation may be thought of as if due to a variationin capacity, or ...of an inductance. The total oscillator circuit reactance may be considered to consist of two parts. That part physically present asin the variable tuning condenser and the tuning coil of circuit I3, and that part reflected by the control circuit of vtube 20. 'I'he latter is a function of the intermediate frequency carrier frequency suplator has a' high degree of smbmty and win notv vary appreciably. This change in frequency is Aconverted into a change in voltage appearing across the diode resistors 24 and 26 by means of plied by I'I and I8, and its magnitude varies A with the shift from the intermediate frequency mid-band frequency. It will be noted that the plate of control tube 20 is coupled into the high potential side of the tank circuit, and that the grid of the control tube is excited by a voltage which is 90 out of phase with that appearing across the tank circuit. The plate current in the control tube is then likewise out of phase with the tank voltage, and accordingly,the control tube 20 looks like a reactance tothe tank circuit. A similar reactance tube has been described in Travis United` States application #4,793 led February 4, 1935, and in Travis United States application #19,563, filed May 3,

1935. New and improved reactance tubes have' been 'described in my United States application #209,919 filed May 25, 1938, Patent No. 2,250,095, July 22, 1941.

The sign. of the reactance presented to the tank circuit by the control tube depends upon the nature of the impedance across which .is d'eveloped a voltage tobe fed to the input electrodes of the control tube. The magnitude of thev reflected reactance depends upon the automatic frequency control bias impressed upon the control grid of control tube 20, as this varies the mutual conductance, and proportionately the magnitude of the plate current, of the control tube. In Figure 1 there is shown Onemethod of obtaining the out-of-pliase excitation for the grid of the control tube,'and it will be noted that this is accomplished by taking the voltage across condenser C3 in series with resistance Ra coupled with the tank coil I3 and anode 3|. 'I'he effect of this arrangement is to produce an inductive effect across the circuit. As the plate current of tube 20 is decreased the 'shunting effect of the reflected inductance lis decreased and the effective inductance of-` the ,circuit I3 will therefore increase. `This results in'a rise of oscillator frequency. When the plate current to 20 increases the effective reactance supplied tocircuit *I3 increases and the frequency ofthe prduced oscillations falls. j

The reactance of the tube 20 between its plate 30 and cathode 25 is connected as shown in shunt to the anode 3l and grid 32 of oscillation generator I0 or effectively in shunt to the frequency determining circuit I3.

When the frequency ofthe oscillator III varies a given amount the intermediate frequency appearing in the plate circuit of tube 30 Awill Vary the same number of cycles since the crystal oscil-A the frequency discriminatilng transformer I1, Il

and the direct coupling C. After passing through either one of the time constant circuits R101 or R102 the energy is passed to the grid 28 of the reactance tube so that the reactance in the plate of tube 20 is changed by an amount sufdcient to compensate for the change in frequency of the oscillator. Thus, the mean frequency of the oscillator is automatically held to its selected mean frequency atV a stability dependent upon the stability ofthe crystal oscillator 20..and of the frequency discriminating circuits C, I1, and I8.

Signal modulation vmay bev applied through jack 35 and transformer. 29 Ato the grid 28 of the reactance tube 20 to vary the tuning of the matic frequency control C and I 'I and I8 `in-l 'cluding the intermediate frequency discrimination circuits. Thus, the' oscillator may be varied at. a rat'e corresponding to the signalling frequency but may not be varied at a rate lower than signalling frequencies so that\the slow drifts of mean frequency are compensated lfor without lation."

impairing the ,applied frequency moduv'hecircuit of Figure2 contains the following vadditional modifications which may be applied to the invention of the application: 1. The use of a line controlled oscillator 30 for the heterodyning oscillator.

' 2. The use of a different form of reactance' I tube 20 for varying ,the tuning ofr the modulated oscillator I0.

3. The use of a limiter action to remove unwanted amplitude modulation from the energy 'applied to the automatic frequency control circuit.

4. Thev .addition of a monitoring circuit to monitor the quality of the frequency modulation.

The line controlled oscillator shown in Fig.

ure 2 and comprising ltube 30 and its associated circuits including resonant line I4 may employ,

the principle of stabilizing the frequency of an oscillator as described by MessrsQC. W. Hansell, and P. S. Carter in the Proceedings of the Institute Alof Radio Engineers, vol. 24, No. 4, April 1926. 'I he title of this article is Frequency control by low power factor line circuits. No novelty is herein claimed as tothe particular typeof line controlled oscillatory to be used.

The reactance tube 20 of Figure 2 utilizes broadly the method .hereinbefore described but differs therefrom in several respects. Here the reactance effect is obtained by applying the feedback voltage from theV plate of the reactance tube to its grid. Here however, new and im-' proved results are obtained by a simple arrangement utilizing suppressor grid modulation as described in my United States application #209,919, filed May 25, 1938,l Patent No. 2,250,095, July 22, 1941.

voltage on 33. The reactance of vCs is made Elements Cs and Ra form thefeedback phase shifting circuit by means of 4phase with the current through CeRs.

`modulated wave applied at its input.

large compared to the resistance of R: for the frequency used so that current through the circuit is largely determined by the condenser. Since the current is capacitively reactive, it leads the voltage by 90. The voltage in Ra will be in Consequently, the voltage across Ra is 90 out of phase with respect to the voltage across CeRa and the required phase relation ls obtained. The condenser Cs may represent the capacity between 33 and 28 or may represent a separate condenser supplementary said capacity. Modulating potentials are applied to the suppressor grid 29.

The detector tube 50 of Figure'2, is separate instead. of being included as a part of the pentagrid converter as shown in Figure 1. 50 is a diode so that diode limiting may be utilized at this point in the circuit. Briefly the principle of operation of the diode limiter 50 consists in applying the voltage to be limited at a level which is high compared to the voltage which is applied for heterodyning. Since a diode detector is a linear detector it detects the envelope of the The detected output is proportional to the mean voltage of the modulated wave and the percentage of modulation. Thus, Ih-=kEM, where Ih is the heterodyne output, k a yconstant of the circuit and tube, E the mean Voltage and M the percentage of modulation. When the diode is used as a heterodyne detector, the modulated Wave is due to variations in strength of the carrier wave and local oscillator wave which are applied at the input. Hence the strongest of the two voltages is modulated up and down by the addition and subtraction of the weaker. The percentage of modulation is therefore proportional to the ratio between `the carrier and local oscillator voltages, or M .-:Eo/Ec where Eo=oscillator voltage and Ec=carrier voltage in the case where the carrier voltage is the strongest. Since the mean voltage will be equal to Ec, the heterodyned output will be:

x Eo Ih-kEcXEcltEo Consequently with the carrier voltage stronger than the local oscillator voltage, the heterodyne output is dependent upon the oscillator voltage 'the-heterodyne output is constant for all values of signal greater than equality with the local By varying thetrength of the locall oscillator. oscillator the point where limiting 'begins may be adjusted. Thus, Venergy of relatively high only. This is the feature which allows the de-y Terman, page 228.

circuit enables the monitoring of the detected frequency modulation which will appear across the diode resistors. This modulation appears at the diode resistors by virtue of the transformer lli/I8 which converts the frequency modulation toamplit'ude modulation for detection by thediodes 40, 4 I. Thus, the quality of -frequency modu'lation obtained due to the application of modulation potentials, through jack and 'transformer 29, may be observed and defects noted. It will be noted that in Figure 2 oscillator tube I0 utilizes a somewhat different circuit than the oscillator of Figure 1. The oscillator of Figure 2 utilizes a grounded-plate circuit in place of the grounded-grid circuit of Figure 1. Both of these circuits are particularly well adapted to the placing of a reactance tube across the oscillator tuning circuit. Any oscillator circuit with one side of the circuit at radio frequency ground potential Would be as well adapted as these two types y v of circuit. For instance, a Meissner circuit would be so adapted. For a diagram of the Meissner circuit see the book Radio Engineering by F. E. The Reversed Feedback and Tickler Feedback circuits on thevsame page of that book would be likewise suitable.

ThisI type of. circuit may be used to produce phase modulation of the type wherein frequency 'modulation is corrected to phase modulation by the use of a particular value of time constant R1Ci Or RiCz. v value as would enable the production of frequency modulation. and RiCz might be of such a wso amplitude from oscillator I0 designated herein-l before as the carrier is fed'from the tuned circuit I3 by means of link' coupling 5I to the tuned circuit 52 of the diode detector-limiter 50. Energy from the tank circuit I4 of the linecontrolled heterodyning oscillator 30, I4, etc., designated hereinbefore as the local oscillator is fed from circuit I4 to the diodel input circuit 52 by means of-link coupling 53. The differential transformer I 'I, I8 and diode detectors 40, 4I, are the same as described in Figure 1.`

. The audio monitoring circuit consisting of amplifier tube 54 feeding throgh an output transformer 56 to jack 55 is coupled to the diode resistors 24 and 26 at a point preceding the time constant `circuit RiCi or C2 through which the automatic frequency control energy is fed. This tively a phase modulatio value as would enable the production of phase modulation. Hence, by switching switch S either phase or frequency modulation may be chosen. The time constant R'iCz for phase modulation would `be -of such a value as to allow the automatic frequency control to partially compensate for the frequency deviations of the lower signalling frequencies. Hence, a frequency deviation would be produced which would be proportional to the audio frequency applied. A frequency modulation with such roportionality is effec- For frequency modulation, the time constant of RiCi would be adjusted to a Malue which is subs tantially-greater than the period of the lowest modulating frequency. Thus for aclowest modulating frequency of 50 cycles, the time vconstant .would be substantially greater than 1,4,0 of a second-a value of about`1/5 second'would be satisfactory. With this value oftime constant, R1C1 would sufllciently attenuate frequencies higher than about 50 cycles so that only frequency changes having a rate of change less than 50 cycles per second would be allowed to pass from the frequency discriminating detector circuit to the reactance tube. Hence if a modulating potential having a frequency of 50 cycles or higher were applied through transformer 29, the frequency of the oscillator I0 would be modulated f in accordancewith -the potentials andthe automatic frequency control acti-on would not oppose the modulating potentials. r

'In the case of phase modulation production, the time constant of RiCz is adjusted so that the modulating'potentlals of lower frequencyare opposed by the automatic frequency control action.

As-I have shown in my United States applicationy #618,154 filed June 20, 1932, Patent No, 2,229,640, January 28, 1941, a phase modulated wave is equivalent to a frequency modulated wave whose modulating potentials have been distorted with respect to their frequency characteristic such Thus, RiCi might be of such a f I that the vdegree of frequency modulation is pr portional to the frequency ofthe modulating potentials. That is', all thatisnecessary to produce phase modulation on a frequency .modulation transmitter is to a'dd a correctioncircuit to the modulating potentials Isuch that each modulating frequency is applied with an amplitude propo'rtional to its frequency. Hence, the low frequency modulating potentials must be attenuated with respect to the high frequency modulating poten` tials or the high frequency modulating potentials increased in amplitude in accordance with their frequency. This may be done with the aid.

of an audio correction network inserted in the leadssupplying the modulating potentials, but it may be also done inthe novel manner of the present invention. The method ofthe present invention is to make the time constant R102 ofv such a valu'e that the automatic frequency con- -trol action tends to reduce the degree of fretransformer 29 and the degree of frequency mod ulation for that modulation frequency will be reduced. For a higher modulation frequency the attenuating effect of RiCi would be greater so that there would not be as much opposition to the modulating potentials and a greater degree of frequency modulation would result.` 'I'his sort of characteristic with a degree of frequency mod- V ulation which risesas the modulation frequency rises is just what isdesired .to produce a phase modulation wave. for a time constant of e second, C2 would be adjusted to have a reactance at the lowest modulating frequency of about 1/5 the resistance of R1.' Such an adjustment would attenuate a modulation frequency of cycles by a ratio of 5,:1 and a modulation frequency of 5000 cycles by a ratio of 500:1. The lower modulating frequencies would therefore be attenuated less than the higher modulating frequencies so'that the de- Thus,finstead of adjusting gree of frequency modu ation of the lower modulation frequencies wo d be reduced to a' greater extent than that of the higher and a phase modulated wave would result. Various degrees' and i' characteristics of attenuation could be obtained by substitutingother elements for RiCa, for instance a resistance in series with a condenser could be substituted for C2 in order to decrease the rate at which the attenuation increased with frequency. The ltime constant circuit RiCi or RiCz might also consist of other elements besides resistance and capacity so as to producethevdesire/d correction circuit to correct the frequency modulationinto phase modulation. This type of quired. I

It will be apparent tq thoseskilled in the art that the-crystal oscillator energy might be supplied from a separate unit which contained some form of stable oscillator with or without frequency multipliers. A multi-.grid tube .such .as the 6L7 could then be used `as the frequency converter in place of the pentagrid converter 30 shown in Figure l.

, In a modification I utilize the principle of inverse feedback to reduce the distortion and still obtain the increased stability which the circuits of `this application have. In order \to. apply inverse feedback to transmitters, the procedure employed is to receive some of the energy to -be radiated and apply this rectified energy to the audio system at a phase such that the energy fed back/opposes the energy applied as modulating potentials. Thus, in the case of the use of inverse feedback on an amplitude modulation transmitter, some of the transmitted 'fenergy' I is rectified and fed back into the audio system with lthe polarity adjusted so that the feedback and applied modulating potentials oppose. This type of degeneration or inverse feedback has the property of reducing ldistortion due to non-linearity and also tends to flatten out the frequency characteristic. The principle .of inverse feedback has been described by. H. S. Black in an article entiled Stabilized feedback amplifiers in vol'. 53, page 114; January 1934 of Electrical Engineering; (also in 'the January 1934 issue of 'Ihe Bell System Technical Journal). A notable use of the principle in a broadcast transmitter is described by Poppele,

.Cunningham and Kishpaugh in an article entitled, Design and equipment of a fifty-kilowatt broadcast station for WOR in the Proceedings ofthe I. R. E., vol. 24, No. 8, August 1936.

In my application of inverse feedback to a frequency modulatiorr transmitter, I receive some of the energy from -the frequency modulated oscillator on a` frequency]- modulati'om receiver consisting of a frequency discriminating circuit and rectiiiers/and then feed this rectified energy in series with the modulating potentials at' a polarity which opposes the modulating potentials. This procedure increases the required amplitude of the modulation potentials fora given degree of frequency modulation, but this' may be compensated for by inserting additional amplification in the modulating potential circuits.v

The circuits of Figure 1 and Figure 2 are ideally adapted to the application of inverse feedback,

The only change necessary is to eliminatethe time constant consisting of R1, Cior C: or else substitute a time constant of such value that it will pass all of thefrequencies -in the modulation band equally well.- 'I'he latter adjustment is preferable since the time constant then tends to preventradio frequency energy from feeding back, but allows the modulating potentials to be fed back without attenuation. With either of A these adjustments the detected output of the phase modulatorwouid be very advantageous .where high degrees .of phase modulation are rel diodes 40 and 4 I therefore, corresponds to the envelope" i of frequency modulations on the 'energy from I3.v Thus, when this detected output is fed backuto oppose the applied modulating potentials, inverse" feedback is obtained with its accompanying advantages.

frne operation of these circuits with their time constant circuits removed,l or adjusted so that they do not attenuatethe modulating potentials. 1s in mostrespects the same as that of the circuits with a higher value of tigne constant present.` 1

e. only difference being inthe requirement of modulating lpotentials ofhigher amplitudes'. Additional audio freqiency amplifiers can. be added to the equipment connected at 35 to 29. This requirement would be compensated for vby a reduction .of distortion with.respect to liearity and frequency characteristc.- Since itis rather difiicult to obtain a frequency deviation which isinexact linear accordance Witlrthe modulating potentials, the improvement in thisA respect isy greatly'to be desired.

' In the phase modulation modication of these circ1,1its,inverse feedback is also used, but thel adjustments possible one as previously described where the time constant circuit allows only slow variationsv to pass and thereby correct for slow drifts in mean frequency, and the other in .which the time constant is removed and the feedback potentials are allowed to oppose the modulating potentials, but the modulating potentials are increased a compensating amount.

The second modification concerns the elimination of the step of heterodyning. For instance in the case 'of Figure 1, tube 30 and its oscillator circuit may be removed and transformer I 1, I 8 may be tuned tothe frequency of oscillator I3, I0 and fed directly by energy from oscillator I0. This arrangement is not as preferable as the heterodyne arrangement since the frequency discriminating circuit operates at a high frequency and the reduced selectivity inherent to the higher frequency will reducethe degree/of overall stability obtained. However, the advantages inherent to the inverse feedback are still present and the system also has more exibility with regard to change of operating frequency. The specific arrangement of the circuit of Figure 1 to eliminate the stp of heterodyning is to eliminate tube 30 and all of its associated elementsfincluding the primary tuned circuit I1. Tuned circuit I8 is then coupled directly to I34 and condenser C *35 tapped on coil I3 at the same position that lead I6 is tapped at present. y

The arrangement of Figure 2 may, in like man- .ner, vbe modified to employ reversed feedback,

mean frequency control and frequency or phase modulation by a tuned radio frequency control receiver /reactance tube control modulator etc., by

coupling I8' directly to the circuit I3 and directly to. circuit I3 by condenser C and removing 50, 5I, 53, I1', I4' and 30 and its circuits. In this case the operation is the same as described herenbefore except that the discriminator circuit comprising C and I8' is operating at a higher frequency and is dimensioned accordingly.

The principle of this invention utilizes inverse r feedback for all of its adjustments. The differwould becapable of being frequency modulated at a relativelyhigh'rate, but which would automatically reduce frequency deviations or drifts .which were of a relatively slow rate, i. e., of a rate lower than that of the lowest modulating frequency. In order toproduce phase mo'dulation .with this adjustment an audio correction circuit such as disclosed in my United States application #608,333 med Api-i1 30, 1932, now

Patent #2,085,739 dated July 6, 1937, may be inserted in the modulating potentials path as indicated by dotted lines at I0 in Figures 1 and 2, between the modulating potential source and the transmitter.

(2) lWith a time constant of a value somewhat less than 1/5 second which would allow the automatic frequency control action to reduce the degree of modulation on the lower modulation frequencies. This adjustment would automatically correct the modulating potentials so that the frequency deviations of the lower mod ulat-A ing frequencies would be reduced and the resulting wave would then be a phase modulated `wave and the distorting network 4D may be removed or adjusted to pass all potentials in like manner.

`(3) With either no time constant at all or with a value of time constant which would not the modulating potentials fed to the transmitter would have to have a correction circuit 40 applied which would make their amplitudes proportional to their frequency in the known manner of correcting afrequency modulation transmitter for phase modulation emission as disclosed in my aforesaid application.

I claim: f

1. .In a wavelength modulation system, a high frequency wave generator l,the mean frequency of 'operation of which may vary slowly, means for producing potentials characteristic of slow variations in the mean frequency of operation of said generator, substantially inertialess means connected with said generator and actuated by said potentials and operating without mechani- Acal movement for preventing slow variations in the frequency of operation of said wave generator and means for modulating the length of the charge tube means connected with said generator and with said first named electron discharge tube means and actuated by said produced po'- tentials for preventing slow variations of the mean ,frequency of operation of said wave generator and additional means cooperating with saidv second electron discharge tube means for modulating the length of the wave generated by said generator in accordance with controllingv potentials of a frequency greater than said slow variations. i

3. -In a wavelength modulation system, a high frequency wave generator the mean frequency of operation of which may -vary slowly, means connected with. said generator for'producing poten-- tials characteristic of the slow variations in the mean frequency4 of operation thereof, substantially inertialess means operating without mechanical movement coupling said first means to said generator and actuated by saidvproduced potentials for preventing said slow variations of the mean frequency of'operation' of saidwave generator and means for modulating the length of the wave generated by said generator in accordance with controlling potentials of a frequency greater than said slow variations, said two means having a part in common. 4. In a frequency modulation system, a signailing wavegenerator, an oscillation generator, substantially inertialess means operating without ,mechanical movement actuated by oscillatory energy from said generators for preventing slow variations of the mean frequency of operation of said signalling wave generator and means for modulating the frequency of said signalling wave generator in accordance with controlling potentials of a frequency greater than said slow variations, said two means having a part in common.

5. In a phase modulation system, a main oscillation generator, an auxiliary oscillation generator, means coupled with both of said generators and actuated by energy produced by said main generator and bysaid auxiliarygenerator slow frequency variations. and means for impressing additional controllingl potentials on said variable -tube reactance for, contfolling the frefor preventing slow variations Vof the frequency of operation of said main generator, means for i modulating the frequency of said main generator in accordance with signal potentials of a frequency greater than said slow variations, and means for modifying said signal potentials in accordance with their frequency.

6. In aI wavelength modulation system, anoscillation generatorof controllable frequency, an oscillation generator of substantially fixed frequency, means for producing potentials characteristic of slow variations of the frequency of' said rst generator relative to the frequency of said second generator, substantially inertialess means operating without mechanical movement energized by said potentials for controlling the frequency of lsaid first generator to oppose said slow variations, and means for impressing modulating potentials on said second named means for modulating the frequency of said first generator.

7. In a frequency modulation system, an oscillation generator of controllable frequency, an oscillation generator of substantially xedfrequency, means for producing potentials characteristic ofslow variations of the frequency of said first generator relative to the frequency ofquency ofsaid 'first generator in accordance with signalling potential.

A10. The method of phase modulating oscillator energywhich includes the `steps of, producing oscillator energy ofl controllable frequency the frequency of which may vary slowly, utilizing said slow variations to produce frequency-4 changes of said oscillator energy which oppose any tendency of said produced oscillatory energy to-change slowly in frequency, producing modulating potentials the amplitudes of which vary substantially in accordance with the frequency thereof and producing variations in the fre- .quency of4 said produced oscillatory energy in' accordance with said produced modulating poltentials of .a frequency greater than said slow changes. e

11. In a modulation system, a high frequency -oscillator comprising an electron discharge device having its electrodes coupled in oscillation producing circuits, a second high frequency oscillator comprising a second electron discharge tube having electrodes connected in oscillation l producing circuits, frequency stabilizing means in the oscillation producing circuits of said last named discharge tube, combining means coupled to both of said circuits, a frequency discriminating' circuit coupled with said combining said second generator, substantially inertialess means operating without mechanical movement energized by said potentials for controlling the frequency of said first generator to oppose said" Jslow variations, and means for impressingsignailing potentials on said last substantially -inertialess named means for controlling thefrequency of said first generator at signal frequency.

-8. In a phase modulation system, an oscillation generator of controllable frequency, an oscillation generator of substantially fixed frequency, means for producing potentials charac--l teristic of slowA variations of the frequency of said rst generator relative to thefrequency of said second generator, means energized by said potentials for controlling the frequency of said first generator to oppose said slow variations, means for modifyingv signal potentials in accordance with their frequency, and means for impressing) said modied'signal potentials on said second named means for controlling the frequency of said rst generator. i

9. In a frequency modulationsystem, an elec tron discharge tube oscillation generator of controllable frequency, an electron discharge tube oscillation generator of substantially fixed frequency, electron discharge tube means for promeans,f a rectifier having Vinput electrodes cou` pled with .said frequency discriminating circuit, said rectifier having output electrodes, a frequency control circuit including an electron discharge tube reactance coupling the output electrodes of said rectifier to said first oscillator, and a source of modulating 'potentials coupled to said last named coupling.

l2. In a modulation system,.a high frequency t oscillator comprising an electron discharge device having its electrodes coupled in oscillation producingcircuits, a second high frequency os-A cillator comprising a second electron discharge tube having electrodes connected in oscillation producing vcircuits and havingv additional electrodes.' frequency stabilizing means in the oscillation producing circuits of said last named electron discharge tube, a, couplingv between one of said additional electrodes and said first .named electrpn 'discharge device, a frequency discriminating circuit coupled with one ofsaid additional electrodes, a diode rectifierhaving input electrodes coupled with said frequency discriminating circuit, said diode rectifier having output electrodes, a reactance tube having input and output electrodes coupled by phase shifting reactances to said first 'named electron discharge devicea coupling between the output electrodes of said diode rectifier and the input electrodes o'f said reactance tube and a source'of modulating potentials coupled to said last namedpoupling.

13. In a modulation system, a high frequency oscillatorvcomprising an electron discharge device having its electrodes coupled in oscillation producing circuits, a second high frequency oscillator comprising a second 'electron discharge ltube having electrodes connected in oscillation ducing potentials characteristic of slow varia-n ,tions of the frequency of saidl first generator vproducing circuits and having additional electrodes, frequency stabilizingmeans in the oscillast named coupling, and a source of modulating potentials coupled to said last named coupling. Y

14. In a modulation system, a first oscillation generator comprising an electron discharge tube having electrodes coupled in oscillation producing circuits, asecond oscillation generator comprising an electron discharge tube having electrodes coupled in oscillation producing circuits, frequency stabilizing means in said last named circuits', a rectifier having input and output electrodes coupled "in an alternating current input circuit and an alternating current output ciri lcuit, a coupling between said alternating input -circuit and both of said generators, a frequency discriminating circuit coupled to said output alternating current circuit,'a rectifier ,having input electrodes coupled to said frequency discriminating circuit, said rectifier having output electrodes, a reactance tube having input and output electrodes, phase shifting reactances coupling the input and output electrodes of said reactance tube to said rst named oscillation generator, a circuit including time control elements coupling the output electrodes of said rectifier to an electrode in said reactance tube, and means for impressing modulating potentials on said last named circuit.

15. In a modulation system,` a rst oscillation generator comprising a discharge tube having electrodes coupled in oscillation producing circuits,`a second oscillation generator comprising an electron discharge tube having electrodes coupled in oscillation producing circuits, frequency stabilizing means in said last named circuits,

, an amplitude *limiting diode rectifier having introdes coupleddiiferentially tosaid frequencyl discriminating circuit, said diode rectifiers having output electrodes, a reactance tube having input and output electrodes, phase shifting reactances coupling the input and output electrodes of said reactance tube to said first named oscillation generator, a circuit includingftime control elements coupling the output electrodesof said diode rectier to an input electrode in said` reactance tube, and means for impressing modulating potentials lon said last named circuit.

variations in the frequency of operation of said main generator, means for controlling the reactance of said variable reactance in accordance e with said electrical variations for opposing said ling the gain o f said reactance tube in accordslow variations of the frequency of operation of said main wave generator, means for modulating the length of the wave generated by said main wave generator in accordance with signalling potentials of a frequency greater than said slow variations, said twoV means having a part in common, and monitoring means connected with said first named means. y

17. In a wavelength modulation system, an oscillat-ion generator of controllable frequency, an

oscillation generator of iixed frequency, means for producing potentials characteristic of slow variations 0f the frequency of said rst generator relative to the frequency of said second generator, reactance tube means coupled to said first generator and energized by said potentials for controlling the frequency of said first generator to oppose said slow variations, means for impressing modulating potentials on said last named means for controlling the frequency of said first named generator in accordance with said modulating potentials, and monitoringI means coupled ,with said i-lrst means.

18. In va wave -length modulation system, an electron discharge device generator having electrodes connected in oscillationl generating and wave length controlling circuits, a reactance tube having an anode, a control electrode and a cathode, means coupling said anode and cathode yin shunt to a portion of said generator circuits to icontrol, the wave length of the oscillations generatedmhase displacing circuits coupling the anode and control electrode of said-reactance tubeto said generator to impress on said control electrode and anode phase displaced 'oscillatory potentials to produce in said reactance tube a reactive eiect between said anode and cathode and in shunt to a portion of said generator circuits, a wave length discriminating circuit coupled with said generator circuits, rectifying means coupled Vwith said discriminating circuit for producing potentials characteristic of slow changes in the wave length of the oscillations generated in sani discharge device and circuits, means for controlance with said produced potentials to thereby vstabilize the operation bf said generator, and means for controlling the gain of said reactance tube in accordance with modulating potentials to modulate the wave length of said generated wave.

19. A system asy recited in claim18 wherein said means for controlling the gain of said reactance tube comprises an auxiliary electrode in said reactance tube connected by a circuit including time constant elements to said'rectier.

20. In a wave length modulation system, van

' electron discharge device having a plurality' of 16. In a wavelength modulation system, a main wave generator,- an auxiliary wave generator, a variable reactance of the reactance tube type associated with said main generator to control the frequency of operation thereof, means for deriving wave energy from said generators and producing electrical variations characteristic of slow electrodes connected in oscillation generating and wave length stabilizing circuits, a reactance tube having an anode, a cathode, a control electrode and an auxiliary electrode, means coupling said` anode and cathode with said generator circuits to control the wave length of the oscillations generated, phase displacing circuits cou`- pling the anode and control electrode vof saidA produced between the anode and cathode of said reactance tube and in said oscillation generating coupler, a source of modulating potentials, 'and connections between said source of modulating potentials and the auxiliary electrode and cathode of said reactance tube for impressing modulating potentials onthe auxiliary electrode and cathode of said reactance tube to control the s 2,279,669 Y' circuits to which said anode and cathode are gain of said reactance tube in accordance with f said modulating potentials t thereby control the and wave length stabilizing circuits, a source of modulating potentials, a reactance tube having an anode, a cathode anda control electrode, phase displacing means coupling the anode and 4control'electrode of said reactancetube to said generator circuits, means coupling the cathode of said reactance tube to said generator circuits, means coupling said source of modulating potentials to an electrode insaid reactance tube to control the gain thereof and thereby modulate' the length of the oscillations generated, means trode, a coupling between the anode of said tube and said oscillation producing circuits, a phase displacing circuit coupling the control electrodel of said -tube to said oscillation producing circuits, a coupling between the output of said discriminatoncircuit and the control electrode of said tube and means for varying the impedance of said tube at signal frequency to thereby-modulatel the frequency of the oscillations produced at signal frequency.

24. In a frequency modulation system the combination of, Y.means for producing oscillations to be modulated in frequency at signal frequency comprising an electron discharge device having its electrodes coupled in oscillation generating circuits to generate oscillations the length of which may vary slowlyin an undesired manner,

coupled with said generator circuits for producing potentials characteristic of said length modulations of said oscillations, and means for y impressing said produced potentials on an electrede of said reactance tube to control the gain of said tube in a sense to oppose saidaforesaid gain control to thereby impove the linearity of said wave length modulating action.

22. The method of signalling by means of wave energy of carrier frequency and controllable wave length which may vary in mean frequency in an undesired manner slowly, and potentials characteristic of signals, which includes the steps of, producing potentials characteristic of the slow variations in the mean frequency of said wave energy, utilizing` said produced potentials to control the mean frequency of said wave energy in a sense to `oppose said slow variations modifying the amplitudes of said signal potentials as a function of their frequency, modulating the length of said wave energy in accordance with said modined signal potentia rectifying the modulated wave energy to produce potentials characteristic of the wave length modulations on said wave energy, and additionally controlling the length of said wave energy inaccordance with the nature of said last produced potentials means for -stabilizing the frequency of the oscillations generated in said circuitsl comprising an electron discharge tube having a plurality of Ielectrodes including a controlling electrode, means coupling two of said electrodes tn din'erent points on said oscillation producing circuits, a phase shifting circuit coupling said controlling electrode to a third point on said oscillation producing circuits, a frequency discriminator circuit having an input coupled to said oscillation producing circuits and an output coupled to an electrode in said electron discharge tube, said frequency discriminating circuit having a characteristic such that the voltages 'at the output thereof vary in accordance with slow changes in the frequency of the oscillations produced in Said oscillation producing circuits and means for varying the impedance of said dischargeC tube at `signal frequency to modulate" the frequency of the stabilized oscillations. f

in a sense opposite to saidmodulating action to improve the linearity of said wave energy in* accordance `with said potentials characteristic of Signals.

of modulation of the length 23. In a frequency modulation system a highy frequency oscillator comprising an'electron discharge device having its electrodes coupled in oscillation producing circuits to produce oscillation the'mean frequency of which may vary slowly in an undesired manner, a frequencydiscriminator circuit having an vinput and having an output, the characteristic of said discriminator circuit being such that the output thereof varies in accordance with variations in the mean frequency of wave energy applied to the input thereof, means for impressing voltages the frequency of which gary in accordance with said slow variations inthe frequency of the oscillations produced in said ,device on the `input of said frequency discriminator circuit, an electron discharge tube having a plurality of electrodes including a cathode, an anode and a control elec.-

vin saidy oscillation circuit on -tlre frequency discriminator 25. In a frequency modulation system a high frequency oscillator comprising an oscillation ciriit in which oscillating electricall voltages the equency of which .may change slowly in an undesired manner are caused to flow, a frequency discriminator circuit having an input and having an output, the characteristic of said discrimi, nator circuit beingsuch that the potential at the output thereof varies in accordance with Vvaria-- tions in the mean frequency of oscillatory energy applied to the input thereof, means for impressing voltages the frequency of which vary in accordance with'saidlslow variations in the frequency of the electrical voltages caused to now input of said circuit, ,an electron discharge device having a plurality of electrodes including a cathode, and a controlling electrode, a coupling between an electrode-of said device and said oscillation circuit, a phase displacing circuit connecting thfcontrol electrode of said device to said oscillation circuit, av coupling be-l --tween` the output of said discriminator circuit and an electrodeof said device for impressing gain controlling' voltages'from the output of said discriminator circuit on sai last namedelectro'de and means fo/rrvarying the, gain of said device at signal frequeiclo thereby modulate the fre'- quency -of th'e oscillations produced at signal fre-- quency.

26. In a' 'frequency-modulating system,'^an'os cillation generator having a-.fiequency-determining resonant circuit, means to vary the enecti've value of one of the reactive components of said circuit in accordance withsign'al impulses to produce frequency-modulation of the generated oscillations, means for stabilizing tlie mean frequency of the oscillations 'comprising a tuned news. hish frequency wave detector coupled to the resonant circuit and connected to react on the said means to produce frequency-modulation, in such direction as to counteract departures from the mean frequency, and means preventing said stabilizing means from exerting control of the frequencyr` except with respect to relatively slow variations in frequency whereby the stabilizing means does not prevent frequency modulation in accordance with the signal impulses. i 27. In a frequency-modulation system, an osmeans for modulating the length of the wave energy flowing in said circuit including a react- 'I ance tube having an anode, a control electrode,

an auxiliary grid, and a cathode, connections coupling said anode and cathode with said first mentioned circuit to control the wave length of the high frequency wave energy flowing therein,

a phase displacing, network coupling the control cillation generator including a resonant circuit for determining the frequency, a space discharge tube having a plate impedance, means including said tube for varying the tuned impedance of said circuit to vary the frequency in accordance with variations in the plate impfedance of said tube, means for controlling the plate impedance of said discharge tube in accordance with signals,

and means for independently controlling the plate impedance of said tube in response to slow departure of thefrequency from normal.

28.'In a frequency-modulating system, an oscillation generator having a resonant circuit for determining the frequency, -means comprising a control tube for varying the resonant frequency of said circuit at signal frequency to produce frequency-modulation, and other means including said control tube for counteracting slow departures from normal of the frequency of the generated oscillations.

electrode fof said reactance tube to said first mentioned circuit, a source of waves of signalling frequencies, and connections coupling said last named source to said auxiliary grid and cathode for controlling the gain of said reactance tube with waves from said source at signal frequencies to thereby modulate the length of the wave energy.

29. In a frequency modulation system, an os- A cillation generator, means for controlling the frequency of said generator, a reactance tube connected to said generator, means for varying at signal frequency the effective reactance of said reactance tube whereby to vary the frequency of said oscillation generator at signal frequency,

means for utilizing one portion of the oscillations of varied frequency for transmittal, means for reducing the mean frequency of another portion of the frequency varied oscillations of said generator, and means for utilizing the energy `of reduced mean frequency for controlling the mean frequency of operation of said oscillatlongenerator, 30. In a frequency modulation system, a tube oscillation generator having an anode, a cathode and a control grid, means for connecting said r anode, cathode and control grid to high frequency circuit for producing high frequency oscillations, a reactance tube having an anode, a cath` ode and a control grid, means connecting the anode of the reactance tube and the anode of the oscillator together, means for feeding oscillations fromsaid oscillator in phase shifted relation to the .control grid of said reactance tube,

vmeans connecting the cathode of the oscillator to the cathode of the reactance tube, means for varying the voltage across the grid and cathode of the reactance tube at signal frequency for varying the frequency of said oscillations produced by said oscillation generator, means for -energizing an antenna with energy derived from one portion of said frequency varied oscillations, means for producing the jrequency of another portion of said frequency varied oscillations, and

32. A transmitter comprising a high frequency generator having a resonant circuit which cont trols the frequency of operation of said generator, means for varying the resonant frequency of s aid circuit to vary the frequency of operation of said generator inaccordance with modulating waves including, an electron discharge tube having an electron emitting electrode, 'an electron receiving electrode and an electron flow control electrode, means for impressing voltage from said generator substantially in phase quadrature relation on said electron receiving and electron flow control electrodes respectively, whereby a reactive effect is produced insaid tube, means I' coupling a pair of electrodes of said tube to said resonant circuit to supple ent the reactance thereof -by said reactive effect, and means for controlling the gain of said tube by modulating potentials to modulate lsaid reactive effect, a substantially constant frequency crystal controlled oscillator,- and meansresponsive to waves derived from said generator and from said crystal controlled oscillator to automatically frequency control said generator,

33. In a transmitter, a wave generator having.

a resonant circuit, a source of modulating potentials, means to frequency modulate the output of said generator with potentials derived from said source of modulating potentials including, an electron discharge tube having an electron emitting electrode, an electron receiving electrode and an electron flow control electrode, circuits for impressing alternating'current from said generator on said electron receiving electrode and on said electron flow control electrode substantially in phase quadrature with respect to the alternating current impressed on said electron receiving electrode, a circuit coumeans for utilizing the energy of reduced-frer quency for'controlling the effective or average mean frequency of said oscillation generator.

31. In a wave length` modulation system, a cira cuitrwherein. substantially constant frequency mshiuency'waye energy to be modumed energy amplifying and transmitting 4means coupled pling theimpedance between the cathode and electron receiving electrode of said tubeto said resonantcircuit, connections between said tube and source of modulating potentials for controlling the gain ofsaid tube in accordance with erator, land means preventing saidestabiliging?!Vv means from exerting control of tgie freqfcy of operation of said generator except with respect to relatively slow variations in frequency whereby" the stabilizingjmeans does not prevent frequency modulation of said generator in accordance with the modulating potentials.

34. A transmitter comprising a generator, a source of modulating potentials, reactance tube means for frequency modulating the output of 4said generator with waves derived from said source of modulating potentials, means for transmitting a portion of the modulated wave energy derived from saidlgenerator, a substantially constant frequency oscillator, means for beating together waves derived from said oscillator and generator, a discriminator circuit, means for feeding beat wave energy to said frequency discriminator, a rectier coupled to said discriminator, and means including said reactance tube for utilizing the output of said rectifier to control the mean frequency of operation of said generator.

35. In combination, a source of modulating waves, means for amplifying said modulating waves, a high frequency generator having a frequency controlling reactance, an electron discharge tube having an electron emitting electrode, an electron now control electrode and an electron receiving electrode, circuits including phase displacing elements coupling said generator to said electron flow control electrode and said electron receiving electrodes to supply to said electrodes alternating currents displaced in phase substantially 90 degrees whereby a reactive eiect is produced in said tube, circuit means coupling said tube to said reactance to supplement the said reactance, a circuit coupling said amplifying means to said tube for controlling said reactive effect by said amplifying means for varying the frequency of said generator in accordance with said amplified modulating waves, means for transmitting a portion of the outputof said generator, Va combined oscillator and converter, means for feeding wave energy derived from said generator to said converter, means for vfeeding the output of said combined oscillator and converter to a frequency discriminator circuit, a rectier coupled `to said discriminator circuit, and means for utilizing the output of said rectiner to control the mean frequency of operation of said generator.

36. A transmitter comprising a. regeneratively Jconnected high frequency generator, a source of modulating potentials, a modulation potential amplier for amplifying said potentials, a reactfance tube circuit coupling said amplifier and said. generator whereby the output of said generator is varied in frequency in accordancewith said modulating potentials, means for feeding a.

portion of wave energy derived 'from said generator to a transmitting output circuit, a crystal controlled high frequency oscillator, means to combine wave energy derived from said oscillatorv and waves derived from said generator to produce waves of varying frequency differing in freerator, means for feeding said differing frequency waves to a frequency discriminating network, means forNrectifying'wave energy modified by cuit in accordance with signal potentials to produce wave length modulation ofthe generated oscillations, and circuit connections for stabilizing the mean wave length of the oscillations generatedincluding a tuned detector coupled to the resonant circuit and connected to the reactance tube to control the said reactance tube to produce wave length modulation of said generator in such a direction as tocounteract departures from the mean wave length generated, said circuit connections including said tunedI detector connected to said reactance tube also controlling the effective value of one of the reactive components of said resonant circuit in a degenerative sense to improve the linearity'of said wavelength modulation.

38. In a wave length modulation system, an oscillation generator having afrequency-deter-r mining resonant circuit, a reactance tube coupled withsaid resonant circuit and controlled by signals to vary the effective value of one of the reactive components of said circuit in accordance with signals to produce wave length modulation ,of the generated oscillations, 4means and circuit connections for stabilizing the mean frequency` of the generated oscillations including a tuned detector coupled to the resonant circuit and'connected to` said reactance tube to control the same to produce wave length modulation in such a direction as to counteract departures from the mean frequency, said circuit connections also serving for producing potentials which are modified replicas of the signals and for controlling said reactance tube by said last produced potentials in a sense opposite to the control thereof by said signals.

, quency from the waves generated by said gen- 39. In a wave length modulation system, an oscillation generator including a resonant circuit which determines the wave length of the oscillations generated, a space discharge tube having an-impedance between two ofV its elec-y trodes,` means including said tube impedance for varying the tuned impedance of said resonant circuit to vary the wave length of the oscillations Ygenerated in accordance withvarlations -in the n ytrol tube for varying the resonant frequency of said resonant circuit at signal frequency to produce wave length modulation of the oscillation generated, and additional circuits including said Ycontrol tube for counteracting slow departures.,

from nprmal ofthe wave length of the .generated said discriminating network, and meansffeeding said rectified wave energy to said reactance tube circuit for controlling the meanfrequency of operation of 'said generator.

, signal potentials to vary the eective value of one of the reactive components ofv said resonant cirv oscillations for degeneratively controlling said wave length modulation of the oscillationsgenerated as a function of the modulated wave envelope.

41. In a wave length modulation system, an

oscillation generator having wave length determining `resonant circuit, a reactance tube excited by voltages derived from said resonant circuit and signal voltages to vary the eiective value of one of the reactive components of said circuit in accordance with Signal voltages to produce wave length modulation. of the generated oscillay tions, and 'apparatus for stabilizing the wave length of the oscillations generated including a tuned detector coupled to the resonant circuit and connected to react onthe said reactance tube to produce wave length modulation in such a direction as to counteract slow departures from the mean wave length and to apply potentials to said reactance tube which oppose said signal voltages to produce degenerationin said reactance tube to improve the linearity of said wave length modulation,

42. A wave length modulation system comprising in combination, means for generating wave energy to be modulated including an electron discharge device including electrodes. connected in wave generating circuits, apparatus for modulating the length of the wave generated by said wave generating circuits including a reactance tube having an anode, a control electrode, an auxiliary grid, and a cathode, connections coupling said anode and cathode with said genv erating circuits to control the frequency of the wavesv generated, voltage phase. displacing ele- -ments coupling the control electroder of said re- "actance tube toA said generating circuits, acircuit coupled with said auxiliary grid ior controlling the gain of said reactance tube at signal frequenof the oscillations generated from the normal frequency and for varying the output impedance of said device as an inverse function of the modulation envelope of said wave length modulated oscillations.

45. In a frequency modulation system, an oscillation generator of-.controllable frequency, a reactance tube connected therewith to control the frequency of `-the oscillations generated thereby, an oscillation generator of substantially nxed frequency, arst circuit for controlling the vreactance of said reactance tube at signal frequency to modulate the frequency of the oscillations generated by said first generator at signal frequency, a second circuit vfor producing potentials characteristic of slow variations and 'variations at signal frequency of the frequency of said first generator relative to the frequency of cyto thereby modulate the length of the generated waves, and a circuitcoupled with said auxiliary grid for impressing thereon potentials which-are substantially an inverse copy of the modulation Ienvelope of said modulated wave;

43. A wave length modulation system comprising in combination', apparatus for generat wave energy to-be modulated including an electron'discharge device havingl a plurality of electrodes connected in oscillation generating circuits, apparatus for modulating the length of the wave generated by said oscillation generator including a reactancettube having an anode, a oontrol electrode, an-auxiliary grid, and a cathode,

connections coupling said anode and cathode with saidgenerating circuits to control the frequency of the oscillations generated, phase displacing elements coupling the control electrode cf said reactance tube to said generating circuits,

,a 'circuit connected with said` auxiliary grid for controlling the gain of said reactance tube at l signal frequency to'thereby modulate the length of the generated waves, and .a circuit forimpressing potentials on analectrodeof said tube which are substantially"y 'verse copy of the modulation envelope of the sa d modulated wave.

44. In a'wave length modulation system, an

csc tion generator including a resonant circuit for determining the frequency of the oscillations generated, an electron discharge device having an output impedance connected in shunt to a portionof said resonant circuit, said electron discharge device having a control grid and an additional grid, moans including said control grid for .varying the tuned impedance of said-circuit to -vary thefrequency ofthe oscillations generated in aoco Q ce with variations in the output impedance'of said device, means including saidad- -ditional;lrid electrode of said tube for controlling said second generator, a third circuit which inciudes part of said rst circuit for controlling the reactance of said reactance tube by said produced potentials for'controlling the frequency of said first generator to oppose said slow variations and for controlling the reactanceA of said reactance tube as an inverse function of the signal modulation on the oscillations generated by said first generator to improve the linearity of said modulation.

, 46. In a wave length modulation system, means for generating wave energy to be modulated including an electron discharge device having electrodes connected in an oscillation generating circuit, apparatus for modulating the length of the wave generated by said v`oscillation generator includingg` a reactance tube having an anode, a

of the modulation envelope of said .modulated Wave. z 4'7. In a wave length modulation system, an

, oscillation generator including a resonant circuit said output impedance of said device in accord- -ance with-31811315120 thereby modulate. the wave length .of the oscillations generated, and circuits connectedwith an electrode of said device forV controlling the output impedance of said device in rsponsevto slow departures of theY frequency for determining the frequency of the oscillations generated, an electron discharge device having an output impedance coupled in shunt toga portion of said resonant circuit, said electron discharge device having a control grid and an additional grid, means including said control grid for varying the tuned impedance of saidrcircuitto vary th'e frequency of the oscillations generated in accordance with variations in the output lmpedance of said device, a modulation circuit coupled to, said additional grid electrode of saidv device for controlling saidoutput impedance of said device in accordance with signals to thereby modulate the wave lengthof the oscillation generator, and a control circuit coupled with said additional grid for varying the output impedance of said device as an inverse function of the' modulation envelope of said wave length modulated oscillations. I r

MURRAY G. CROSBY.v

4cathode, a control electrode, and an additional, 

